1. Field of the Invention
The present invention relates to an optical scanning device for use in an image forming apparatus.
2. Description of the Related Art
In image forming apparatuses, such as laser printers, an optical scanning device is used to optically scan a target surface for scanning, which is photoconductive and photosensitive in nature. More particularly, in the optical scanning device, an optical deflector deflects a light beam emitted from a light source toward the target surface and a scanning optical system focuses the deflected light beam on the target surface. The scanning optical system includes, e.g., one or more fθ lenses that focus the deflected light beam on the target surface in the form of an optical spot. The optical spot is used to optically scan the target surface.
In a typical full-color image forming apparatus, four photosensitive members are arranged along the direction in which a sheet of recording medium is conveyed for image formation. Each photosensitive member functions as a target surface for scanning. Light beams emitted from a plurality of light sources are guided to each photosensitive member for optical scanning with a single optical deflector. Each deflected light beam then passes through one of a plurality of scanning optical systems. Each scanning optical system is arranged corresponding to a single photosensitive member. Upon passing through a scanning optical system, the deflected light beam is focused on the corresponding photosensitive members. In this way, all of the four photosensitive members are simultaneously exposed to light. As a result, an electrostatic latent image is formed on each photosensitive member. The electrostatic latent images are developed into single-color toner images by applying a toner of yellow, magenta, cyan, and black, respectively. The single-color toner images are then sequentially transferred and superimposed on the sheet to form a full-color toner image. The full-color toner image is then fixed on the sheet by applying heat and pressure to obtain a color image.
An image forming apparatus that forms a multicolor image (e.g., a dichromatic image or a full color image) by using a plurality of combinations of an optical scanning device and a photosensitive member is known as a tandem color image forming apparatus. In such a tandem color image forming apparatus, a single optical deflector can be used to deflect light beams toward a plurality of photosensitive members.
For example, an optical scanning device is disclosed in which an optical deflector receives light beams from two sides and distributes the light beams to a plurality of photosensitive members (refer to Japanese Patent Application Laid-open No. H11-157128 and Japanese Patent Application Laid-open No. H9-127443). Moreover, an optical scanning device is disclosed in which a plurality of substantially parallel light beams spaced apart in a sub-scanning direction are deflected by using an optical deflector and focused on corresponding photosensitive members by using a plurality of scanning optical elements arranged in the sub-scanning direction (refer to Japanese Patent Application Laid-open No. H9-54263). Furthermore, an optical scanning device is disclosed in which a plurality of light beams are deflected from one side of an optical deflector toward corresponding photosensitive members via a scanning optical system that includes three scanning lenses L1, L2, L3. From among the deflected light beams, the light beams corresponding to a certain set of photosensitive members pass through the scanning lens L1 while the light beams corresponding to another certain set of photosensitive members pass through the scanning lens L2. All the deflected light beams pass through the scanning lens L3 (refer to Japanese Patent Application Laid-open No. H9-54263, Japanese Patent Application Laid-open No. 2001-4948, Japanese Patent Application Laid-open No. 2001-10107, and Japanese Patent Application Laid-open No. 2001-33720).
In this way, a single optical deflector can be used to deflect light beams to a plurality of target surfaces for scanning. Such a configuration enables to downsize the optical scanning device and reduce the manufacturing cost. Configuring a compact optical scanning device is in line with growing demand for a compact and low-cost image forming apparatus.
To further reduce the manufacturing cost of the optical scanning device, the scanning optical system can be configured to include a single scanning lens (single-lens configuration) instead of two scanning lenses (two-lens configuration). Moreover, by arranging the scanning lens close to the optical deflector, the optical scanning device can be downsized. However, in that case, the scanning optical system functions as a magnifying optical system with respect to the sub-scanning direction. That is, the absolute value of magnification ratio in the sub-scanning direction of the scanning optical system becomes greater than one. In such a magnifying optical system, shape errors or positional errors of optical elements affect the optical performance of the optical scanning device to a great extent. Thus, it becomes difficult to manufacture a compact optical scanning device at low cost while maintaining high optical performance.
For example, Japanese Patent Application Laid-open No. 2006-309090 discloses an optical scanning device in which the scanning optical system has a single-lens configuration. In that optical scanning device, optical performance is maintained by optimizing the positioning of a scanning lens in the scanning optical system. However, because the scanning optical system functions as a magnifying optical system with respect to the sub-scanning direction, shape errors or positional errors of optical elements therein affect the optical performance to a greater extent as compared to a same-size optical system or a reduced optical system in which a scanning optical system has a two-lens configuration.
Meanwhile, an optical scanning device includes a light receiving element that, upon receiving a synchronization light beam via a synchronization optical system, detects a synchronization signal. To facilitate downsizing of the optical scanning device, it is desirable that the light receiving element is arranged at only one of a write start side and a write end side of the corresponding target surface for scanning. Such a configuration is hereinafter referred to as a single point synchronization configuration. In a single point synchronization configuration, when a scanning lens is arranged close to the optical deflector, the synchronization light beam passes through the scanning lens after getting deflected from the optical deflector. In that case, if the shape of the scanning lens varies due to temperature fluctuation, there is a possibility that a scanning position in a main scanning direction on the light receiving element deviates by a large amount (refer to Japanese Patent Application Laid-open No. H11-44857). Such deviation in the scanning position is not easily noticeable in a black-and-white image forming apparatus. However, in the case of a color image forming apparatus of opposite scanning type in which two target surfaces are scanned in opposite main scanning directions by using a single optical deflector, the deviation in the scanning positions causes color shift in the color image thereby deteriorating the image quality. Moreover, if the optical path from the deflecting surface of the optical deflector to the light receiving element has an almost identical optical path length as the optical path from the deflecting surface of the optical deflector to the corresponding target surface for scanning, then it becomes difficult to downsize the optical scanning device.
To downsize a magnifying optical system, an optical deflector needs to deflect a light beam at a wider angle of view with respect to the optical axis in the main scanning direction. For that, it becomes necessary to configure the optical deflector with a large deflecting surface in the main scanning direction. That problem can be solved by using a polygon mirror as the optical deflector. The size of the deflecting surface of the polygon mirror can be controlled by reducing the number of deflecting surfaces and widening the angle of deflection. However, because it is necessary to rotate a polygon mirror at high speed, problems such as heat, noise, energy consumption, and speed-up limitation arise.
A synchronization light beam to be guided to a light receiving element is deflected at a wider angle with respect to the optical axis in the main scanning direction as compared to an angle at which a light beam to be guided to a target surface for scanning is deflected. Thus, it is necessary that optical elements in the synchronization optical system are accurately positioned with respect to the main scanning direction in an optical box.